Synthetic papers, or probably more accurately termed paper-like sheets, comprising a biaxially stretched film of polypropylene as a base layer and monoaxially stretched films of polypropylene containing 8 to 65% by weight of an inorganic fine powder formed on both surfaces of the base layer as paper-like layers have been proposed and come into commercial acceptance in place of conventional pulp papers as described in, for example, Japanese Patent Publication No. 40794/71 and Japanese Patent Application (OPI) Nos. 141339/81 and 118437/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Such synthetic papers have fine pores around the nuclei of the inorganic fine powder and numerous long cracks on both surfaces. They are light in weight and have excellent drying property of a printing ink, adhesion of a printing ink, writing property with a pencil and waterproofness.
To impart better offset printing property, those synthetic papers are used after their surfaces are coated with an aqueous solution of an acrylic copolymer, polyethyleneimine, etc., in an amount of a solids content of from 0.005 to 1 g/m.sup.2 and dried as described in, for example, Japanese Patent Application (OPI) Nos. 10624/75, 161478/75, 40883/73 and 149363/82.
The synthetic papers having such a coated resin layer are used as labels on containers holding liquids such as motor oil and shampoo, blanks of fancy foods, and art paper for maps and posters.
The surfaces of those synthetic papers are rough (a Bekk index, measured in accordance with JIS P-8119, of from 100 to 2,000 seconds) because the paper-like layer is a stretched film of a thermoplastic resin containing an inorganic fine powder and the coated resin layer is as thin as 0.01.mu. or less.
Papers for posters are required to be offset-printed in multicolors with excellent gloss. In particular, in those posters which advertise or publicize package tours and motion pictures or promote the sales of records, automobiles, carbonated drinks, etc., using young female models, multicolor offset prints of strong gloss are preferred in order to create a bright and light-hearted atmosphere.
Such posters have been previously produced by applying offset multicolor prints to the surface of a synthetic paper having the above-described coated resin layer, coating an isocyanate type primer on the printed surface, and then laminating a transparent polyvinyl chloride film onto the primer layer. Because of the surface smoothness of the polyvinyl chloride film on the surface, those posters look as if prints of excellent gloss were made.
The production of those posters having a laminated layer of polyvinyl chloride additionally requires a transportation expense because they come through both printers and laminaters. To the users, direct delivery of the posters from the printers is advantageous in regard to both time and cost.